Featured Research

from universities, journals, and other organizations

How to efficiently merge microdroplets using an electric field

Date:
August 24, 2011
Source:
Institute of Physical Chemistry of the Polish Academy of Sciences
Summary:
In microfluidic devices, small separated droplets flow in a stream of carrier liquid. Occasionally, selected droplets have to be merged to carry out a chemical reaction. This can be greatly facilitated with the use of electric field, through a process of electrocoalescence that has been used industrially in large scale applications. Researchers have discovered the laws governing the process and how to maximize the efficiency of the merging.

Alternating electric field accelerates merging of microdroplets. Images show phases of merging of water microdroplets in hexadecane at voltage 400 V and field frequency 100 Hz. First frame t=0 s, second frame t=0.28 s, third frame t=2.6 s. False colours.
Credit: IPC PAS

In microfluidic devices, small separated droplets flow in a stream of carrier liquid. Occasionally, selected droplets have to be merged to carry out a chemical reaction. This can be greatly facilitated with the use of electric field, through a process of electrocoalescence that has been used industrially in large scale applications. Researchers from the Institute of Physical Chemistry of the Polish Academy of Sciences have discovered the laws governing the process and how to maximise the efficiency of the merging.

Small droplets in emulsions can merge much faster in the presence of an alternating electric field. The phenomenon is called electrocoalescence and is of essential importance for the operation of advanced microfluidic devices, allowing one to carry out chemical reactions in microliter volume or less. A recent study carried out at the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) allows for improved control of electrocoalescence and for optimisation of the course of the process.

Electrocoalescence has been known for over 100 years. Originally, it has been used in the oil industry. Crude oil extracted from the sea floor contains significant amount of water in a form of droplets. Engineers have noticed that in the presence of an alternating electric field these droplets merge quickly and fall on the tank bottom from where the water can be easily removed. "Electrocoalescence has started its career untypically, from large scale applications in the oil industry. In microfluidics the phenomenon has been applied to merging of droplets only within the last few years," says Dr Piotr Garstecki from the IPC PAS.

Microfluidic devices are miniaturised chemical reactors resembling a credit card in size. Chemical reactions take place inside the droplets that are suspended in a neutral liquid flowing through appropriately designed microchannels. The droplets can be very small: from a fraction of a microliter even down to nano- or picoliters.

"The primary challenge in microfluidic techniques is to combine the stability of droplets with the ability to merge them. Stabilization requires covering the interface with a surfactant, i.e., a surface active agent," explains Tomasz Szymborski, a PhD student at the Institute of Physical Chemistry of the PAS. Surfactant molecules are composed of a hydrophilic (water-loving) and a hydrophobic (water-hating) part. If a water droplet is placed in oil, the surfactant covers it so that the hydrophilic parts are immersed in the droplet, whereas the hydrophobic ones remain in oil. "From outside, the droplets covered with surfactant resemble rolled up hedgehogs and have no chance to touch each other. Their stability is increased due to the fact that surfactant molecules repel each other via electrostatic and entropic forces," explains Szymborski.

Problems appear when precisely selected droplets of different reagents are to be merged in a microfluidic device to carry out a chemical reaction. Since recently electric field has been used to induce merging. Electrocoalescence is known for its macro scale industrial applications, yet how the mechanism and efficiency of the process depends on the parameters of the electric field was largely an open question.

The researchers at the IPC PAS observed merging of water microdroplets in a carried liquid, hexadecane. The rate of droplet merging increased in line with applied voltage and the frequency of electric field oscillations. For each voltage there was a limiting frequency, above which the droplets became stable again. "We showed that the merging proceeded at its maximum rate when the electric field oscillated with a frequency close to a threshold one and we found a simple function allowing to estimate the value of the threshold quickly," says Szymborski.

Rapid merging of droplets is related, i.a., to a periodic movement of ions contained in the droplets that are stimulated by an alternating electric field. The ions separate in the oil-droplet interface while charging it electrically. The droplets with opposite charges attract strongly, which results in droplet merging in spite of the presence of stabilizing surfactants. The results of the study suggest that there is a simple relation between the nanoscopic electrostatic screening length and the optimum frequency for merging of droplets.

The data collected at the IPC PAS will be helpful for practical optimisation of processes involving electrocoalescence, both in microfluidic devices and industrial plants. The results will also facilitate formulation of universal laws describing the efficiency of electrocoalescence in non-equilibrium systems such as flowing liquids.

The research has been financed from a TEAM grant from the Foundation for Polish Science and the Iuventus-Plus Programme of the Polish Ministry of Science and Higher Education.


Story Source:

The above story is based on materials provided by Institute of Physical Chemistry of the Polish Academy of Sciences. Note: Materials may be edited for content and length.


Cite This Page:

Institute of Physical Chemistry of the Polish Academy of Sciences. "How to efficiently merge microdroplets using an electric field." ScienceDaily. ScienceDaily, 24 August 2011. <www.sciencedaily.com/releases/2011/08/110824091229.htm>.
Institute of Physical Chemistry of the Polish Academy of Sciences. (2011, August 24). How to efficiently merge microdroplets using an electric field. ScienceDaily. Retrieved October 20, 2014 from www.sciencedaily.com/releases/2011/08/110824091229.htm
Institute of Physical Chemistry of the Polish Academy of Sciences. "How to efficiently merge microdroplets using an electric field." ScienceDaily. www.sciencedaily.com/releases/2011/08/110824091229.htm (accessed October 20, 2014).

Share This



More Matter & Energy News

Monday, October 20, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Gulfstream G500, G600 Unveiling

Gulfstream G500, G600 Unveiling

Flying (Oct. 20, 2014) Watch Gulfstream's public launch of the G500 and G600 at their headquarters in Savannah, Ga., along with a surprise unveiling of the G500, which taxied up under its own power. Video provided by Flying
Powered by NewsLook.com
Japanese Scientists Unveil Floating 3D Projection

Japanese Scientists Unveil Floating 3D Projection

Reuters - Innovations Video Online (Oct. 20, 2014) Scientists in Tokyo have demonstrated what they say is the world's first 3D projection that floats in mid air. A laser that fires a pulse up to a thousand times a second superheats molecules in the air, creating a spark which can be guided to certain points in the air to shape what the human eye perceives as an image. Matthew Stock reports. Video provided by Reuters
Powered by NewsLook.com
Hey, Doc! Sewage, Beer and Food Scraps Can Power Chevrolet’s Bi-Fuel Impala

Hey, Doc! Sewage, Beer and Food Scraps Can Power Chevrolet’s Bi-Fuel Impala

3BL Media (Oct. 20, 2014) Hey, Doc! Sewage, Beer and Food Scraps Can Power Chevrolet’s Bi-fuel Impala Video provided by 3BL
Powered by NewsLook.com
What We Know About Microsoft's Rumored Smartwatch

What We Know About Microsoft's Rumored Smartwatch

Newsy (Oct. 20, 2014) Microsoft will reportedly release a smartwatch that works across different mobile platforms, has a two-day battery life and tracks heart rate. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:

Breaking News:

Strange & Offbeat Stories


Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile: iPhone Android Web
Follow: Facebook Twitter Google+
Subscribe: RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins